1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, in which a liquid developer is used for producing a toner image on a transfer medium.
2. Description of the Related Art
An electrophotographic type image forming apparatus, which produces a developed image by using a liquid developer, has following advantages: extremely fine toner particles of sub-micron in diameter can be used so that a high quality image comparable to that of the offset printing is realized, copying cost is reduced because sufficient image density can be obtained with a small amount of toner, and energy saving is accomplished because the toner can be fixed to a copy sheet at a relatively low temperature. All of those advantages are not obtained with an electrophotographic recording apparatus using a dry developer.
As one method for transferring the toner image formed on a photosensitive member to a transfer medium in an image forming apparatus using a liquid developer, there is a pressure transfer method that transfers toner particles on a surface of a photosensitive member with the aid of adherence of toner particles by pressing the photosensitive member to the transfer medium. In the pressure transfer method, the toner particles are transferred from the surface of the photosensitive member to the transfer medium according as their surface energy and a shearing stress. The transferability of the toner particles from the surface of the photosensitive member to the transfer medium depends on the correlation of the surface energy between the toner particles and the surface of the photosensitive member and the shearing stress between the surface of the photosensitive member and the transfer medium.
The pressure transfer method has an advantage that a high quality image can be obtained because electric disturbance of the toner particles does not occur when transferring is carried out unlike a transfer method using an electric field. Particularly, the pressure transfer method has advantageous in transferring the toner image to the recording medium, such as copying paper under pressure via an intermediate transfer medium because of less transferring load and wide applicability of the recording media.
However, in the pressure transferring method, the intermediate transfer medium requires two antithetical properties that the toner image can easily be ripped off from the photosensitive member while the toner image can easily be transferred to the recording medium. Therefore, there is a less room to select a material for the intermediate transfer medium, and then the permissible zone for transferring becomes narrow.
Furthermore, even if the material for the intermediate transfer medium is selected as appropriate as possible, there has been a possibility of occurrence of inferior transfer particularly at the top edge portion of the image region where the toner image becomes thick, because deterioration of adherence between the toner image and the surface of the intermediate transfer medium takes place, which is caused by the different height between the image region and the non-image region.
To overcome this drawback, Japanese patent publication (Kokai) No. 08-44216 discloses a method wherein a transfer layer of transparent toner is pre-formed entirely on a photosensitive member so as to rip off the toner image easily from the photosensitive member, the transparent toner is then made into a film, thereafter the toner image is formed on the filmed transfer layer, and the toner image is transferred to a transfer material together with the filmed transfer layer. In this transfer method, a thermoplastic resin is employed as the transparent toner, and the transfer layer is made into a film by developing the transparent toner on the photosensitive layer in advance, and then the transfer layer is made into a film by heating and melting the transparent toner. After the toner image is formed on the transfer layer by a conventional electrophotographic process, the toner image is transferred together with the transfer layer by heating again the transfer layer at the transferring step.
However, the transfer method mentioned above has disadvantages in that the properties of the photosensitive member are affected and selection of the photosensitive material is limited, and more over lengthening the life duration of the photosensitive member is prevented, because the transfer method requires a heating process at the transparent toner film making process after the development of the transparent toner on the surface of the photosensitive member. Furthermore, in view of transfer energy, the transparent toner and the photosensitive material have a problem in that they have to be satisfied properties: the toner image and the transfer layer adhere closely together while the transfer layer and the photosensitive member separate easily from each other.
Consequently, it has been expected to realize an image forming apparatus having high transfer efficiency and long life duration of the photosensitive member, yet a high quality image can be obtained effectively, despite the materials of the intermediate transfer medium and the photosensitive member, when the pressure transfer method is adopted to obtain high quality transfer images.